The present invention relates to a method of measuring a height of liquid level (water level) and a liquid-level gauge and more particularly, to a method and liquid-level gauge for measurement of a water level of liquid by measuring a change in force acted by liquid (buoyancy or hydrostatic pressure) on a body movably arranged in the liquid as a change in strain or strain level in an optical fiber.
Liquid-level gauges for measurement of a height of liquid level (water level) based on various principles have hitherto been proposed. For example, an electrostatic capacitance type liquid-level gauge (JP-A-2000-097750 or JP-A-11-030544), a barometric liquid-level gauge (JP-A-2000-088629), a float type liquid-level gauge (JP-A-10-148565 or JP-A-11-326015), an electrode type liquid-level gauge (JP-A-11-023346) and an electric wave type liquid-level gauge (JP-A-10-197617) have been known. Specifically, the present invention contemplates a float or comparable type liquid-level gauge and a barometric or comparable type liquid-level gauge, especially, using an optical fiber.
The float type liquid-level gauge detects a height of a float that ascends/descends as the liquid-level changes and conventionally, it is classified into two kinds of which one uses reed switches and a magnet and the other uses a wire or a tape. Structurally, the former float type liquid-level gauge using reed switches and a magnet has many reed switches that are operated by the magnet as the float ascends or descends. On the other hand, in the latter float type liquid-level gauge using a wire, a measuring wire attached to a float is wound up to calculate a liquid water level from a windup amount.
On the other hand, a barometric liquid-level gauge as exemplified in FIG. 20 has hitherto been employed. In the barometric liquid-level gauge, an air supply pipe 300 having an open lower end is dipped vertically in liquid 200 stored in a tank 100 and compressed air 500 is supplied to the air supply pipe 300 by means of a pump 400. As the supply of compressed air 500 to the air supply pipe 300 proceeds, air fills in the air supply pipe 300 in opposition to a pressure of the liquid 200 stored in the tank 100. When saturated in the air supply pipe 300, the air is discharged to the liquid 200 in the form of bubbles through the lower open end of the air supply pipe 300. At that time, a pressure P in the air supply pipe 300 equals a head pressure xcfx81H when no gas pressure is applied onto the liquid level, the liquid level in the tank 100 is H and density of the liquid 200 is xcfx81. Therefore, the pressure P in the air supply pipe 300 is measured by means of a pressure sensor 700 and a measured value is indicated in terms of liquid level height on an indicator.
In the conventional float type liquid-level gauge, especially, using reed switches and a magnet, however, it is necessary that the magnet be built in the float and a great number of reed switches be incorporated in guide pipes for guiding the float, raising a problem that the number of parts increases and the structure is complicated.
On the other hand, in the float type liquid-level gauge using a wire, many parts such as a windup drum for the wire, a windup motor and a pulley are needed, so that the apparatus is increased in scale and is often troubled because of mechanical windup, thus requiring laborious and time-consuming work for repairs and maintenance.
Further, the conventional barometric liquid-level gauge faces problems that the pump 400 for supplying the compressed air 500 to the air supply pipe 300 is needed and during measurement, the pump 400 must be driven constantly to supply the compressed air 500.
Under the circumstances, the present inventors have studied and conducted experiments in various ways by noticing a change in buoyancy which a body receives from liquid as the liquid level changes in the float type or comparable type (suspension type) liquid-level gauge to confirm that the water level of the liquid can be measured by detecting the change in buoyancy as a change in strain in an optical fiber.
Experiments have been conducted also in the barometric or comparable type liquid-level gauge to confirm that the liquid level height can be measured by displacing a pressure receiving member in accordance with a change in liquid pressure, applying tension to an optical fiber in accordance with the displacement to generate strain in the optical fiber and detecting the strain.
The present invention has been made in the light of the conventional problems and the results of experiments and it is an object of the invention to provide liquid-level measuring method and liquid-level gauge which can measure a water level accurately by using an optical fiber connected to force receiving means movably arranged in liquid to receive force from the liquid and detecting a change in force due to a change in liquid level as strain in the optical fiber or a change in strain therein.
To accomplish the above object, in a method of measuring a liquid level according to the present invention, an optical fiber connected at its one end portion to force receiving means movably arranged in liquid to receive force from the liquid is dipped in the liquid together with the force receiving means, and a change in the force acting on the force receiving means when the liquid level changes is detected as a change in strain in the optical fiber by means of an optical fiber strain gauge connected to the other end of the optical fiber.
The precedently determined correlation between changes in strain in the optical fiber and changes in liquid level of the liquid is consulted on the basis of the detected value to determine a water level of the liquid.
A liquid-level gauge according to the invention comprises an optical fiber, force receiving means connected to one end portion of the optical fiber and movably arranged, together with the optical fiber, in liquid to receive force from the liquid, and optical fiber strain measuring means connected to the other end portion of the optical fiber to detect, as a change in strain in the optical fiber, a change in the force acting on the force receiving means when the liquid level of the liquid changes.
Preferably, the optical fiber strain measuring means is a Brillouin-optical time domain reflector (hereinafter simply referred to as B-OTDR).
According to one aspect of the invention, in a method of measuring a liquid level, a float having a cross-sectional form that is uniform in the height direction and a specific weight value less than that of liquid is dipped in the liquid, the float is supported by an optical fiber in such a manner that an upper end of the optical fiber constantly protrudes from the liquid level, the optical fiber is connected at its upper end to an optical fiber strain gauge, and a change in buoyancy acting on the float as the water level of the liquid changes is detected as a change in strain in the optical fiber by means of the optical fiber strain gauge, thus measuring a water level of the liquid.
According to a second aspect of the invention, in a method of measuring a liquid level, a suspension member having a cross-sectional form that is uniform in the height direction and a specific weight value not less than that of liquid is suspended by an optical fiber so as to be dipped in the liquid in such a manner that an upper end of the suspension member constantly protrudes from the liquid level, the optical fiber is connected to an optical fiber strain gauge, and a change in buoyancy acting on the suspension member as the water level of the liquid changes is detected as a change in strain in the optical fiber by means of the optical fiber strain gauge, thus measuring a water level of the liquid.
In embodiments of the float type liquid-level gauge according to the invention, a liquid-level gauge comprises a float having a cross-sectional form that is uniform in the height direction and a specific weight value less than that of liquid and dipped in the liquid, an optical fiber for supporting the float in such a manner that an upper end of the float constantly protrudes from the liquid level, and an optical fiber stain gauge for detecting a change in buoyancy acting on the float due to a change in water level of the liquid as a change in strain in the optical fiber.
A liquid-level gauge comprises a suspension member having a cross-sectional form that is uniform in the height direction and a specific weight value not less than that of liquid, an optical fiber for dipping the suspension member in the liquid while suspending the suspension member in such a manner that an upper end of the suspension member constantly protrudes from the liquid level, and an optical fiber strain gauge for detecting a change in buoyancy acting on the suspension member due to a change in water level of the liquid as a change in strain in the optical fiber.
In the liquid-level gauge of the present invention, as the liquid level changes, the magnitude of buoyancy acting on the float or suspension member by the liquid changes. Since the cross-sectional area of each of the float and the suspension member is uniform in the longitudinal direction, the magnitude of the change in liquid level is accurately proportional to the change in buoyancy acting on the float or suspension number. Also, the change in strain level in the optical fiber is accurately proportional to the change in buoyancy. In the case of the float, as the liquid water level increases, tension applied to the optical fiber increases in proportion to the increased water level to raise the strain. In the case of the suspension member, as the water level of the liquid increases to increase the buoyancy, tension applied to the optical fiber decreases in inverse proportion to an increase in water level and the strain decreases correspondingly. Accordingly, in either case, when the change in buoyancy as the change in strain caused in the optical fiber is detected by means of the optical fiber strain gauge, the water level of the liquid can be measured from the correlation between changes in liquid water level and changes in strain.
The liquid to be measured is in no way limited to water in the present invention but the invention may also be applied to measurement of the liquid level of various liquids such as oil and medicines and it will be appreciated that xe2x80x9cwater levelxe2x80x9d will be used as a broad word meaning the liquid level height of these kinds of liquids.
According to a third aspect of the invention, in a method of measuring a liquid level by generating strain in an optical fiber in accordance with liquid pressure and detecting the strain to measure a height of liquid level, portions of the optical fiber dipped in liquid are fixed to a fixing member and a pressure receiving member provided in a pressure receiver, respectively, tension is applied to a fiber portion between the fixing member and the pressure receiving member to generate strain in the optical fiber when the pressure receiving member is displaced by a liquid pressure, and the strain is detected by means of an optical fiber strain gauge.
In embodiments of the barometric liquid-level gauge according to the invention, a liquid-level gauge comprises an elongated optical fiber, a pressure receiver having a pressure receiving member displaceable by liquid pressure, fixing members for fixing the optical fiber, and an optical fiber strain gauge for detecting strain in the optical fiber, portions of the optical fiber dipped in the liquid being fixed to the pressure receiving member and the fixing member, respectively, in the liquid and one end of the optical fiber being connected to the optical fiber strain gauge.
In the liquid-level gauge, the pressure receiver has a pressure-tight vessel main body and the pressure receiving member is a piston movable over an opening of the vessel main body to cover it hermetically.
In the liquid-level gauge, the pressure receiver has a pressure-tight vessel main body and the pressure receiving member is a bellows having a pressure receiving plate to hermetically cover an opening of the vessel main body.
In the above barometric type of the present invention, as the liquid level height changes to change the liquid pressure, the magnitude of pressure acting on the pressure receiving member dipped or immersed in the liquid also changes to displace (move or deform) the pressure receiving member. The magnitude of a change in liquid level height is proportional to the magnitude of force resulting from multiplying a change in pressure at a position where the pressure receiver is placed by a pressure receiving area of the pressure receiving member applied with the pressure change. When the pressure receiving member is displaced by a liquid pressure, the optical fiber deforms in proportion to a displacement of the pressure receiving member. In other words, when the liquid level height increases to increase the liquid pressure applied to the pressure receiving member, tension applied to the optical fiber increases in proportion to an increased liquid pressure to increase strain. On the other hand, when the liquid level height decreases to decrease the liquid pressure applied to the pressure receiving member, tension caused in the optical fiber decreases in proportion to a decreased liquid pressure to decrease strain. Accordingly, when the correlation between changes in liquid level height and changes in strain is determined in advance, by detecting a change in liquid pressure acting on the pressure receiving member as a change in strain generated in the optical fiber by means of the optical fiber strain gauge, a liquid level height can be measured accurately from the correlation between the liquid level height and the strain.
In the liquid-level gauge, the vessel main body is provided with a stopper for limiting movement of the pressure receiving member to a predetermined range.
In the liquid-level gauge, the optical fiber has a plurality of portions spaced apart from each other in a direction of depth of the liquid and each fixed by the pressure receiving member and the fixing member.
In the aforementioned barometric type of the invention, the stopper limits the movement of the pressure receiving member to prevent the optical fiber from being broken under the application of excessive tension when the liquid pressure increases.
With a plurality of portions of optical fiber spaced apart from each other in the liquid depth direction and each fixed by the pressure receiving member and fixing member, a liquid level height from a position of each portion can be measured in each portion. For example, when three portions fixed by pressure receiving members and fixing members are provided, a lowermost portion is in charge of measurement of a low liquid level, an intermediate portion is in charge of measurement of an intermediate liquid level and an uppermost portion is in charge of measurement of a high liquid level. In other words, the lowermost portion is for measuring a depth between its position and the intermediate portion, the intermediate portion is for measuring a depth between its position and the uppermost portion and the uppermost portion is for measuring a depth between its position and the liquid level. Accordingly, in case, for example, the intermediate portion is dipped in the liquid during measurement, a depth between the liquid level and the intermediate portion can be measured by detecting strain caused in the intermediate portion and when this depth is added with a depth between the lowermost portion and the intermediate portion, an actual liquid level height can be determined. In case the uppermost portion is dipped, a depth between the liquid level and the uppermost portion can be measured by detecting strain caused in the uppermost portion and when this depth is added with the depth between the lowermost portion and the intermediate portion and the depth between the intermediate portion and the uppermost portion, an actual liquid level height can be determined.